Tweaking PG (again)

Hi.

I had tweaked my PG 8.2.6 with the very kind help of this list a
couple years ago. It has been working fine, until recently. Not sure
if it is after the update to 8.3 or because my DB has been growing,
but the db is very slow now and the cache doesn't seem enough.

~ > free -m
total used free shared buffers cached
Mem: 4051 4033 18 0 6 2576
-/+ buffers/cache: 1450 2601
Swap: 2047 43 2003

Some of the SQL queries that were super fast (indexed with LIMIT 1!)
are now slow too.

What is a good starting point for me apart from checking the slow SQL?
Because almost every SQL is now slow.

I can restart the PG server and it is fast for a little while after
that but then the buffer fills up I think.

It's a CentOS server, Pentium Core2Duo dual processor, 6MB RAM. Same
server runs Apache (low mem consumption), MySQL (for really small web
stuff, not much load) and PGSQL (huge load). I can devote a lot of
memory to PG, no problem.

Autovacuum is on but I also manually vacuum the big tables by crontab
-- per hour.

This one is not a huge DB, about 5GB right now. The tables are as such:

relname | rowcnt | inserted | updated | deleted
---------------+---------+----------+---------+---------
books | 8622136 | 1852965 | 938229 | 16304
checkout_count| 261317 | 9834 | 116664 | 1225
subscribers | 10180 | 1267 | 79623 | 0
interesting | 4196 | 53 | 54774 | 0
pg_statistic | 411 | 0 | 43104 | 0
books_deleted | 896 | 16350 | 0 | 11473
users | 62865 | 2428 | 2493 | 0
pg_attribute | 1844 | 1322 | 575 | 1321
(8 rows)

Below are my CONF settings:

listen_addresses = 'localhost,*'
max_connections = 300
shared_buffers = 330MB
effective_cache_size = 512000
max_fsm_relations = 100
max_fsm_pages = 300000

work_mem = 20MB
temp_buffers = 4096
authentication_timeout = 10s
ssl = off
checkpoint_warning = 3600
random_page_cost = 1

autovacuum = on
autovacuum_vacuum_cost_delay = 20
vacuum_cost_delay = 20
autovacuum_naptime = 10
stats_start_collector = on
stats_row_level = on
autovacuum_vacuum_threshold = 75
autovacuum_analyze_threshold = 25
autovacuum_analyze_scale_factor = 0.02
autovacuum_vacuum_scale_factor = 0.01

Any pointers or advice MUCH appreciated! THANKS.